Combine Erbe di Sicilia, EVOO, salt, pepper, and grated garlic in a zip-lock bag or shallow dish. Coat coppa steak on all sides. Refrigerate 12–72 hours.
Wash marinade off completely. Let steak come to room temperature uncovered for 90 minutes. Pat extremely dry.
Heat Dutch oven over medium-low. Add a thin film of EVOO. Lay steak fat-side down. Cover tightly.
Cook 30 minutes undisturbed. Flip 4 times in succession with no resting between flips. Cook until fork tender. Remove and refrigerate until fully cold.
Slice cold into 1.5–2 inch steaks.
Heat cast iron over high heat until smoking. Add a thin film of EVOO. Sear cold slices hard — 2–3 minutes per side — until a deep crust forms. Pull at 140°F. No rest. Straight to plate.
Return Dutch oven to residual heat. Add minced shallots. Turn heat off. Let shallots sweat in their own liquid. Season lightly with salt.
Add sherry. Deglaze, scraping up all the fond. Reduce by half.
Add demi-glace and simmer 2–3 minutes (or add stock and reduce 10–12 minutes).
Off heat: whisk in room-temperature butter and Dijon if using. Finish with a pinch of Erbe di Sicilia. Season with salt.
Spoon sauce over coppa slices. Finish with flaky salt.